Yup, there's the issue about slashers, but also the fact that after a lot of testing I found that HW2 beam damage looks like it's the damage spread over the duration of the beam, but it actually tends to be significantly variable depending on what you're shooting it at. In particular, it can vary in ranges of +/- 20% depending on the health of the target. It's very weird. I suppose it's a genuine bug in the HW2 engine.
In any case, I need to do more tests on both FSO and HW2 until I successfully manage to accurately emulate the correct beam behavior.